1. Field of the Invention
The present invention relates to a toner for use in developing an electrostatic image. In addition, the present invention also relates to a method for preparing the toner, and to a developer including the toner.
2. Discussion of the Background
Image forming methods in which electrostatic images and magnetic images formed by electrophotographic image forming apparatus and electrostatic recording apparatus are developed with a developer including a toner to be visualized have been conventionally used. For example, electrophotographic image forming methods typically include the following processes:    (1) an electrostatic latent image is formed on an image bearing member (i.e.,. a photoreceptor);    (2) the electrostatic latent image is developed with a developer including a toner to from a toner image on the image bearing member;    (3) the toner image is transferred onto a receiving material via an intermediate transfer medium; and    (4) the toner image is fixed on the receiving material upon application of heat and/or pressure thereto.
Toner for use in developing electrostatic images are typically colored particles in which a colorant, a charge controlling agent and other additives are included in a binder resin or are present on a binder resin. Methods for preparing toner are broadly classified into pulverization methods and suspension polymerization methods.
The pulverization methods typically include the following processes:    (1) a colorant, a charge controlling agent, an offset preventing agent and other additives are kneaded with a melted thermoplastic resin serving as a binder resin to be uniformly dispersed therein;    (2) after being cooled, the kneaded mixture is pulverized; and    (3) the pulverized mixture is classified to prepare a toner.
The pulverization methods have an advantage in that the resultant toner has a combination of medium-level properties, but have a drawback that raw materials used for preparing the toner are limited. For example, the mixture prepared by melting and kneading toner constituents has to be pulverized and classified with conventional pulverizers and classifiers. Specifically, the kneaded mixture has to be brittle enough to be pulverized by conventional pulverizers. Therefore, when a kneaded mixture is pulverized, the resultant power tends to have a broad particle diameter distribution. In order to produce images with good resolution and half tone properties, the particle diameter of toner particles is preferably from 5 μm to 20 μm. Therefore, fine particles having a particle diameter less than 5 μm, and coarse particles having a particle diameter greater than 20 μm have to be removed from the resultant powder, resulting in serious decrease in yield of the toner in the classification process. In addition, it is difficult for the pulverization methods to uniformly disperse a colorant and a charge controlling agent in a thermoplastic resin (i.e., a binder resin). Uneven dispersion of such toner constituents adversely affects the fluidity, developability, durability and image qualities of the resultant toner.
In attempting to remedy the drawbacks of the pulverization methods, toner preparing methods using suspension polymerization have been proposed and practically used. It is well known to produce toner by polymerization methods. For example, a method in which a toner is prepared by a suspension polymerization method is used. However, toner prepared by such a suspension polymerization method has a poor cleanability. This is because the resultant toner particles have a spherical form. When images having a low image area proportion are formed using such a toner, a background development problem in that toner particles remaining on a photoreceptor without being removed therefrom in a cleaning process are transferred onto a non-image area of a receiving material, resulting in occurrence of background fouling is hardly caused. However, when images having a high image area proportion (such as pictorial images) are formed using such a toner or when a large amount of toner particles remain on a photoreceptor due to machine problems such as paper jamming, the background development problem is caused. In addition, another problem which occurs is that toner particles remaining on a photoreceptor even after a cleaning operation contaminate a contact charging roller which charges the photoreceptor-while contacting the photoreceptor, resulting in deterioration of charging ability of the charging roller.
In attempting to solve this problem, Japanese patent No. 2,537,503 discloses a method in which resin particles prepared by associating resin particles prepared by emulsion polymerization are used for a toner. However, toner particles prepared by emulsion polymerization methods include a large amount of surfactant therein and/or on the surface thereof even when the particles are fully washed. Therefore, the toner has drawbacks in that the charge quantity of the toner greatly changes depending on environmental conditions, and the toner has broad charge quantity distribution, thereby causing the background development problem. In addition, a problem in that the charging roller and developing roller used for an image forming apparatus together with the toner are contaminated with the surfactant remaining on the surface of the toner, resulting in deterioration of the charging ability of the charging roller and developing ability of the developing roller occurs.
When a release agent is further associated with particles prepared by such a method, the release agent is incorporated inside toner particles, and thereby good offset resistance cannot be imparted to the toner. Specifically, there is a case where the particulate resin, particulate release agent and particulate colorant are adhered to a portion of toner particles in a concentrated manner, or the materials are hardly adhered to a portion of toner particles. Therefore, a problem in that concentrations of toner constituents such as the resin, release agent and colorant in toner particles widely change occurs. Accordingly, it is impossible for the toner to stably produce good toner images for a long period of time. In addition, due to uneven distribution of the resin particles on the-surface of the toner particles, the toner has a high fixable temperature, namely, the toner has insufficient fixable temperature range.
On the other hand, toner used for contact heat fixing methods is required to have good releasability against heating members of the fixing devices used for fixing images of the toner (i.e., the toner is required to have a good offset resistance). The offset resistance of a toner is typically improved by a method in which a release agent is added to the toner so as to be present on a surface of the toner particles. In attempting to improve the offset resistance of a toner, published unexamined Japanese patent applications Nos. 2000-292973 and 2000-292978 disclose toners in which resin particles are not only included in toner particles but also unevenly distributed on the surface of the toner particles. However, as a result of the present inventors' study, the toner has a high fixable temperature, namely, the toner has insufficient low temperature fixability (i.e., poor energy-saving property).
Because of these reasons, a need exists for a toner having a good combination of cleanability, low temperature fixability, and offset resistance without contaminating image forming members such as charging members, developing members and fixing members.